Recently, there is an increasing demand for a semiconductor device having a high processing speed and a large storage. To meet the demand, semiconductor devices are manufactured having a multilayer wiring structure which realizes an increased wiring density. In such a semiconductor device having a multilayer wiring structure, each layer is formed with a predetermined conductor pattern. The conductor pattern comprises conducting portions such as metal wirings, metal plugs and pads. The metal plug (which comprises a metal material filled in a connection hole) functions to electrically connect two different layers to each other. To attain a much higher density, there is a need for decreasing the width of metal wirings. For this purpose, the metal wiring needs to have a low resistance and a high anti-electromigration property. Therefore, metal wirings and plugs made of Cu or a Cu alloy, which is a good conductor and has a high anti-electromigration property, are being developed.
The damascene process is known as a process for making metal wirings or plugs of Cu or a Cu alloy in a multilayer wiring structure. The technique of the damascene process is described, for example, in C. W. Kaanta et. al., VMIC Conf. Proc. 8, P.144 (1991).
In the damascene process, one layer including metal wirings and plugs is formed as follows. First, an insulating film is formed on a substrate, and wiring grooves for later housing metal wirings and/or connection holes for later housing plugs are formed in the insulating film by etching. Subsequently, a wiring material is deposited in the wiring grooves and the connection holes, thereby forming metal wirings and plugs in a complete product. At this time, the wiring material is deposited also at the portions on the insulating film where the wiring grooves and the connection holes are not formed. The wiring material deposited on the insulating film is removed through the polishing process by CMP. The polishing process flattens the insulating film so that the upper surface of the insulating film is flush with the upper surface of the metal wirings and the plugs formed in the insulating film, thereby providing predetermined conducting portions. In this way, in the normal damascene process, the wiring grooves and the corresponding connection holes are formed individually with respect to each layer by the wiring material loading step and the following CMP step.
On the other hand, the dual damascene process is also known. In this process, wiring grooves are formed in a first insulating film whereas connection holes opening toward the wiring grooves are formed in a second insulating film arranged under the first insulating film. After the wiring material is loaded simultaneously with respect to the wiring grooves and the connection holes, the CMP process is performed with respect to the first insulating film.
In the polishing by CMP, the surface of a substrate is polished for flattening using a slurry containing silica or alumina for example. Subsequently, the substrate surface is polished using pure water as required. In polishing, the surface of the semiconductor substrate is contaminated by silica or alumina itself or by metal of the polished metal wirings or plugs. Such contamination of the semiconductor substrate by metallic impurity influences the electric characteristics of the semiconductor, deteriorating the reliability of the device. Therefore, a cleaning step for removing the metallic impurity from the semiconductor substrate surface need be performed after the CMP step.
In the cleaning step, cleaning is performed using a predetermined chemical cleaning liquid to remove the metallic impurity on the substrate surface. After the cleaning step, a rinsing step is performed using pure water to remove the cleaning liquid. To remove the metallic impurity, the chemical cleaning liquid needs to have a dissolving power for metal. Conventionally, however, the conducting portions exposed at the layer surface after the polishing step corrodes in the cleaning step due to the strong metal dissolving power of the cleaning liquid.
For preventing an electronic component or the like from oxidizing or corroding during the cleaning, JP-A-4-40270 discloses a technique in which an electronic component is cleaned using pure water of a decreased dissolved oxygen concentration. However, such cleaning using pure water cannot remove the metal contaminant after the CMP step. As another method for preventing the oxidization or corrosion of an electronic component orthelike, JP-A-7-60209, JP-A-10-128253 and JP-A-6-318584 each discloses a technique in which an electronic component is rinsed with pure water of a decreased dissolved oxygen concentration after the cleaning step to remove the cleaning liquid. However, the technique disclosed in these documents relates to the rinsing liquid for use after the general cleaning process of electronic components, and does not assume the existence of such a cleaning liquid having a metal dissolving power as that used in the cleaning step following the CMP step.
On the other hand, JP-A-10-72594 discloses a technique for suppressing the corrosion which occurs through the cleaning step following the CMP process. In this technique, the cleaning step is performed using a cleaning liquid containing organic acid including carboxyl group, and a complexing agent such as EDTA. The use of organic acid, which has a relatively low metal dissolving power, can suppress the corrosion at the surfaces of the metal wirings. Further, according to JP-A-10-72594, owing to the existence of the complexing agent, the cleaning ability of the cleaning liquid is not degraded even by the use of organic acid having a relatively low metal dissolving power.
However, even when the cleaning is performed using organic acid having a relatively low metal dissolving power for the cleaning liquid, the wiring material of the conducting portions is partially lost, i.e. corroded at some locations on the layer after the cleaning and rinsing steps following the CMP process. The inventors have found that such local metal corrosion occurs in the rinsing step, not in the cleaning step.
FIG. 9 is a schematic view illustrating the local corrosion of a metal wiring conventionally occurred in the rinsing step. Shown in FIG. 9 is part of one layer after the CMP process and the subsequent cleaning and rinsing steps, in which a predetermined conductor is formed as embedded in an insulating film 100. The conductor pattern comprises a pad 101, metal wirings 102, 103 and metal plugs 104 as conducting portions formed in a same layer. The metal wiring 102 is electrically connected to the pad 101 to provide a set of conducting portions in the conducting pattern. The metal wiring 102 is locally corroded at the edge portions. Although the pad 101 is actually corroded at the edge portions, the corrosion is not illustrated for simplicity, because local corrosion in such a large area does not provide a significant problem. One metal wiring 103 and another metal wiring 103 are separated from each other within the same layer, but electrically connected to each other to provide another set of conducting portions. The electrical connection between these two metal wirings 103 is provided by a metal wiring 105 electrically connected to the both metal wirings 103 via the metal plugs 4 formed in the underlying layer.
The inventors have found that local corrosion occurs at the edges of a conducting portion having a relatively large area, and at the edges of a set of conducting portions electrically connected to each other to have a relatively large area in a same layer. Specifically, as can be seen at the edges of the metal wiring 102 in FIG. 9, local corrosion develops to a considerable degree at the edges of a conducting portion having a surface area of no less than 500 μm2, and at the edges of a set of conducting portions electrically connected to each other to have a surface area of no less than 500 μm2 in a same layer. The reason why the local corrosion does not occur at the edges of the metal wirings 103 of FIG. 9 is that the set of conducting portions including the metal wirings 103 does not have a surface area of no less than 500 μm2 in the layer shown in FIG. 9. The local corrosion during the rinsing step occurs even at a set of conducting portions that consists of wirings only and does not include a pad, if the set of conducting portion has a surface area of no less than 500 μm2 in a same layer.
Metal corrosion in an aqueous solution occurs as local cell reaction, which is the combination of cathodic reduction reaction which occurs because hydrogen ions or dissolved oxygen in the solution act on the metal surface as oxidizer, and anodic dissolution reaction in which metal is oxidized to dissolve in the aqueous solution. For example, in the case where the metal is Cu and the oxidizer is dissolved oxygen, the anodic dissolution reaction is represented by the formula (I), whereas the cathodic reduction reaction is represented by the formula (II). In a metal surface, the point where the cathodic reaction is likely to occur is called a cathodic reaction active site, whereas the point where the anodic reaction is likely to occur is called an anodic reaction active site. The distribution of the two kinds of active sites varies depending on the physicochemical state of the metal surface, i.e. the lattice structure and the concentration of the contacting solution and the like.Cu→Cu2++2e−  (I)1/2O2+H2O+2e−→2OH−  (II)
The CMP process and the accompanying cleaning and rinsing steps are carried out using an aqueous solution. Therefore, in the case where the metal wirings are formed of Cu for example, the corrosion of the Cu wirings occurs as local cell reaction provided by the combination of the anodic reaction (I) and the cathodic reaction (II) When the anodic reaction (I) and the cathodic reaction (II) occur evenly on the surface of the wirings, the Cu wirings corrode generally uniformly. On the otherhand, when each of the two kinds of reaction occurs locally, the dissolution of Cu occurs locally at the anodic reaction active sites, resulting in local corrosion, i.e. local loss of the Cu wirings. JP-A-10-72594 described before discloses the use of organic acid having a low metal dissolving power together with a complexing agent instead of the conventionally used inorganic acid having a high metal dissolving power. Since the driving force of the organic acid for the anodic dissolution reaction, i.e. the metal dissolving power of the organic acid is relatively low, the use of organic acid as an active ingredient of the cleaning liquid suppresses the uniform corrosion of the metal wirings.
However, even when the cleaning step is performed using organic acid having a low metal dissolving power for the cleaning liquid, the conductor pattern having undergone the subsequent rinsing step suffer from local corrosion of the wiring material at a set of conducting portions electrically connected to each other to have a surface area of no less than 500 μm2 in a same layer or at a conducting portion having a surface area of no less than 500 μm2, as does in the metal wiring 102 of FIG. 9.
During the rinsing step, the concentration of the cleaning liquid remaining on the substrate surface gradually decreases to inevitably pass the ultralow concentration range, finally reaching substantially zero. The inventors have found that the local corrosion is likely to occur when the concentration lies in the ultralow concentration range of 4.70×10−4-1.96×10−7 mol/l. When the cleaning liquid lies in the ultralow concentration range, on the surface of a set of conducting portions electrically connected to each other to have a surface area of no less than 500 μm2 in a same layer or of a conducting portion having a surface area of no less than 500 μm2, the anodic reaction active sites are localized at edges of a metal wiring, i.e. at a narrower portion, whereas the cathodic reaction active sites are localized at other portions of the surfaces of a metal wiring and/or a pad, i.e. at a wider portion. Therefore, the two kinds of reaction differ largely in reaction rate per unit area. Conceivably, this is the reason why the local corrosion develops. The reaction rate per unit area of the anodic reaction (I) is higher than that of the cathodic reaction (II), so that the dissolution reaction locally develops at the anodic reaction active sites, causing local corrosion. The local corrosion of the wiring material is extremely small at the edges of a set of conducting portions which are electrically connected to each other and which have a surface area smaller than 500 μm2 in a same layer.
To prevent such corrosion in the rinsing step, a rinsing liquid may be used which contains pure water and a corrosion inhibitor such as benzotriazole added to the water. However, although the use of such a rinsing liquid suppresses the local corrosion, it provides another problem. That is, when the pure water containing a corrosion inhibitor is used in the rinsing step, only the pure water evaporates in the subsequent drying step, leaving most of the corrosion inhibitor on the surface of the semiconductor substrate. When a next insulating film is laminated on the substrate surface in such a state, the corrosion inhibitor may be sandwiched between the conducting portions and the insulating film. This deteriorates the adhesion strength between the layers, causing the release of the layer at that portion. In this way, the use of corrosion inhibitor in the rinsing step causes another problem and hence is not preferable.
It is also found that the similar problems occur during the polishing using pure water in the CMP process. In the process of replacing the polishing liquid adhering to the semiconductor substrate surface with pure water, the concentration of the polishing liquid gradually decreases to inevitably pass the ultralow concentration range, finally reaching substantially zero. When the concentration of the polishing liquid lies in the ultralow concentration range, problems similar to those of the rinsing step occur. That is, local corrosion occurs at the edges of a set of conducting portions electrically connected to each other to have a surface area of no less than 500 μm2 in a same layer or at the edges of a conducting portion having a surface area of no less than 500 μm2.